The overall objective of the proposed research is to understand the neural representation of the acoustic environment in the brainstem auditory system, particularly the cochlear nucleus and the inferior colliculus. The dorsal cochlear nucleus (DCN) receives both auditory and somatosensory input. The nature of the sensory convergence is the first aim of the proposal. The touch inputs carry information about the status of the muscles that move the cat's pinna. This suggests that the role of the auditory/somatosensory convergence is to coordinate the auditory system's interpretation of pinna dependent cues for sound localization with information about where the pinna are pointing. As such, this system serves as model for the coordination of head movements with sound localization in humans observers. In Aim 1, the properties of the convergence in DCN will be investigated, particularly whether the somatosensory inputs induce cross-frequency auditory processing in DCN, and whether plasticity can be observed in the auditory/somatosensory interaction. Aims 2 and 3 will characterize the representation of complex acoustic environments in the cochlear nucleus and inferior colliculus. In Aim 2, a new information-theoretic measure will be used to study the sensitivity of auditory neurons to particular aspects of the sound stream. With this method, the relative sensitivity of neurons to spectral and temporal features of the stimulus, to localization of sounds in space, and to special features of the stimulus that help in perceptual analysis of complex scenes can be analyzed. An important feature of the method is that representations that are encoded in the coordinated activity of ensembles of neurons can be analyzed using the same methods as single-unit representations. In Aim 3, the nature of the neural representation in particular neurons will be studied using methods that attempt to construct predictive models of neural responses. This aim is based on the hypothesis that the auditory system contains parallel, generally nearly linear tonotopic systems that represent stimuli in a homomorphic fashion and non-linear systems engaged in special analyses of particular features which have a particular biological importance.